Viscous strain gage

ABSTRACT

Metal particles are suspended in an elastomeric compound to produce a conductive strain gage element capable of high elongation which responds with electric signals proportionate to the strain being measured, which strain element is mounted on an elastomeric carrier and encapsulated in a rubbery mass for protection.

United States Patent [mi DuBose et al.

[ March 6, 1973 VISCOUS STRAIN GAGE [75] Inventors: Charles R. DuBose;Harvey A. Jessup, both of Waco, Tex.

[73] Assignee: North American Rockwell Corporation, El Segundo, Calif.

22 Filedz May2, 1969 [2l] Appl.No.: 821,208

[52] U.S. Cl ..338/2, 338/114 [51] Int. Cl. ..G0lb 7/18 [58] Field ofSearch ..338/2, 3, 5, 114; 73/885 [56] References Cited UNITED STATESPATENTS 2,734,978 2/l956 Bulgin ..338/l l4 2,344,642 3/1944 Ruge ..338/3E X 2,621,276 12/1952 H0wland.... ....338/2 E X 3,089,107 5/1963 Dean.338/2 E X Primary Examiner-Benjamin A. Borchelt Assistant Examiner-R.Kinberg Att0rney-William R. Lane and Thomas S. Mac- Donald [57] ABSTRACTMetal particles are suspended in an elastomeric compound to produce aconductive strain gage element capable of high elongation which respondswith electric signals proportionate to the strain being measured, whichstrain element is mounted on an elastomeric carrier and encapsulated ina rubbery mass for protec- Hon.

4 Claims, 3 Drawing Figures VISCOUS STRAIN cxcs BACKGROUND OF THEINVENTION State of the art strain gages, for the most part, aresatisfactory where limited stresses are to be measured or monitored. Forexample, mild expansions and contractions of certain types of metalscan-be accommodated by a semi-rigid resistive type of strain gage.However, a problem exists when movement is to be monitored in a materialwherein the material expands or contracts more than a small percent ofits original size. Certain types of ductile metals can be elongated fromto 50 percent before failure, for example, aluminum, cobalt, copper,lead, nickel, gold, silver, platinum, tin, titanium, zirconium and theirrespective alloys plus hafnium, thorium, depleted uranium and vanadium.Prior art foil resistance strain gages normally have a maximumelongation of about 10 percent. A few high elongation strain gages havebeen developed with limited success. A metallic liquid type strain gage(U.S. Pat. No. 3,304,528) utilizes mercury as the conductive sensingelement. Mercury is extremely temperature sensitive and becomes a solidat a relatively mild cold temperature and tends to gas off" or separateat a mild high temperature causing a loss of electrical continuity.Therefore, the gage has a very narrow environmental tolerance whichseverely limits its application. Another type of high elongation gage(U.S. Pat. No. 3,124,769) is a helical type gage wherein the sensingelement is disposed on one or both sides of a helical support coil. Thistype of gage, while it lends itself to high elongation when compared tothe semirigid gage, still is limited in its application due to itsrelative bulkiness and the difficulty in calibration of the device.Still another type of gage designed for high elongation is known as aflexible strain gage, (U.S. Pat. No. 3,205,464) and the object is tomeasure a small segment of the strain since the basic gage is incapableof high elongation. The problem again is the difficulty in calibration.

Therefore, it is an object of this invention to provide a highelongation strain gage.

More specifically, it is an object of this invention to provide aviscous strain gage of low modulus capable of an elongation ofapproximately I00 percent or more of its basic length which varies inelectrical resistance proportionateto its length.

I SUMMARY OF THE INVENTION The viscous strain gage of this invention isa composite structure made up of the following materials; a first layerof a low modulus non-conductive polymer compound acts as a gage carrierproviding a base for a bead of an elastomeric gage-forming compound. Thegage forming compound comprises a natural or synthetic rubber thoroughlyimpregnated with a'quantity of conductive metal particles, such as anoble metal, throughout its length. Conductive wires are attached toboth ends or the gage so that the gage can be connected to suitableelectrical strain measuring instrumentation. Finally, the entire gage isencapsulated with a rubbery compound of low modulus to protect the gage.

Advantage of the highly sensitive gage over the prior art is thecapability of 10.0. percent elongation, its insensitivity to ambienttemperature variations and the ability to. measure large strains onelastomers without apelastomer.

DESCRIPTION OF THE DRAWINGS The above noted objects and furtheradvantages of the instant invention will be more fully understood uponstudy of the following detailed description in conjunction with thedetailed drawings in which:

FIG. 1 is a view of the strain gage attached to a test specimen which isundergoing a strain;

FIG. 2 illustrates the sequential steps in fabricating the strain gage;and,

FIG. 3 is a section taken along lines 3--3 of FIG. 1.

Referring now to FIG. 1 the viscous strain gage generally designated as10 is fastened to a workpiece or test specimen 11 that is to besubjected to a strain. The gage is fastened to the workpiece along itsentire length by suitable means such as an adhesive type of rubbercement 22 (FIG. 3). The workpiece 11 is shown in two positions in theillustration. The test specimen 11 is shown at rest in phantom line andin a strained condition in solid line 11. The specimen can beconstructed of any material from a high to a relatively low modulus forexample, from steel to plastic. Strain can be measured accurately onrelatively low modulus material by gage 10 because the gage structureitself is so weak that it cannot change the modulus of the testspecimen. The relative rigidity of prior art strain gages inherentlychanges the strain imparted to the specimen which results in inaccuratespecimen responses. As can be seen, the workpiece 11 is bowed severelycausing a large lateral displacement along the length of the strain gage10. The displacement is measured resistively and signals indicative ofresistive values are conducted from the gage by electrical wires 17 atopposite ends of the gage. The wires are connected to instrumentation(not shown) which records or measures the changes in resistance of thegage. The strain transmitted from the specimen 11 to the gage 10 variesproportionately as a function of the length of the gage. Normally aconventional Wheatstone bridge arrangemen may be employed as part of theinstrumentation.

Examples of non-metallic material that can be strain tested by thepresent invention include the following; any of the elastomer or rubbercompounds that are used to manufacture rubber products such as tires,tubes, gaskets, bushings, as well as solid rocket propellant motorsfound in the aerospace industry. Flexible plastics can be strain testedby the low modulus gage 10, for example; polyethylene,acrylonitrile-butadienestyrene resin, polyacetal, polyallomer,chlorinated polyether, chlorinated polyvinyl chloride epoxy, nylon,polypropylene olefin copolymer, polystyrene, polyvinyl alcohol,polyvinyl butyral, polyvinyl formal and polyvinyl chloride compounds.

FIG. 2 is a perspective view of the various stages of fabrication of thepreferred embodiments of the invention from stage A through D.

The first stage A consists of dipping a sand blasted glass microscopeslide mandrel 13 in I-Ieveatex L-266 latex solution. I-Ieveatex L-266latex is a commercially available product of l-leveatex Company, aDivision of the Firestone Tire and Rubber Company. The compound is aprevulcanized latex compound manufactured by heating natural latex inthe presence of zinc oxide, sulfur accelerators and antioxidants. Thecompound is as follows:

Parts by Weight The above constituents are used (where the solids isindicated) in the form of water dispersions.

By dipping the slide 13 into the above described compound of liquidlatex, a coating of approximately 0.010 inches is obtained which is thegage carrier 12. The dipped mandrel 13 supporting carrier 12 is then airdried generally for 8 to 25 hours at 70 to 75 F to drive off water. Thecarrier 12 is now ready for the second stage B wherein the active gageelement is prepared. The active gage generally designated as 16 consistsof, for example, 50 parts of Heveatex L-266 (14) having dispersedtherein 150 parts of 340 mesh silver powder 15 (99.99 percent pure) plus50 parts of a ammonia in water solution. The metal particles can be froma mesh size of 200 to 400 and can be a conductive metal other than anoble metal, such as nickel, lead or tin. The compound is mixed togetherfor approximately 30 minutes to obtain thorough particle dispersion anda homogeneous mix thereby readying the conductive material 16 for finalshaping. The gage 16 is then affixed to the carrier 12 therebycompleting stage B. The dimensions of the gage 16 are, for example,one-half inch long, two-tenths of an inch wide by 0.020 inches thick. Bykeeping the completed active element relatively small, its accuracy andutility is enhanced. The heretofore described active element is a 25 ohmgage suitable for any material to be strain monitored. The 25 ohm gage16 has an exponential resistance curve that is proportionate to thestrain. The gage 16 is then affixed to the carrier 12 thereby completingstage B. V

In stage C, electrical leads 17 are prepared for attaching to both endsof the gage 16 at points 18 and 19. The conductive lead wires 17preferably are 99.99% pure silver to assure good conductivity andcompatibility with the silver particles in the active gage element 16.If a different base metal for the lead wires 17 is to be utilized theyare treated in the following manner: The ends of the conductive leads 17are coated with an adhesive such as CHEMLOK 234, a commerciallyavailable product of the Highson Company, a Division of LordCorporation. To assure electrical conductivity and chemicalcompatibility of the adhesive 20, four parts of silver particles(200-400 mesh preferred) are mixed with one part of CHEMLOK 234. Thecomposition applied to the ends of conductive leads 17 thereby preventspossible corrosive interaction between dissimilar metals when silverparticles are used in the conductive element 16. Other commerciallyobtainable rubber-to-metal adhesives may be employed. They typically arechlorinated elastomers with carbon black fillers, sulfur curatives andisocyanate cure accelerators. After the leads 17 are affixed to the ends18 and 19 by the composition 20 an additional quantity of thesilverloaded adhesive is applied over the top of the leads which helpsto reinforce the-wire connection points.

The composite is again air dried for 8 to 25 hours at a room temperatureof from to F to drive off the adhesive solvents.

Stage D further includes the following: the gage element 10 affixed tothe glass slide mandrel 13 is immersed in HEVEATEX L-266 latex longenough to build up a protective encapsulating coating 21 ofapproximately 0.050 inches thick. The viscous gage element 10 is thenair-dried 8 to 25 hours at an ambient temperature of from 70 to 75 F.After this initial drying period the completed gage is then placed in anoven at a temperature between 215 to 235 F for 35 to 55 minutes forcuring. The gage 10 is removed from the furnace and cooled to roomtemperature. The gage is subsequently lifted from the slide mandrel 13by carefully cutting the latex carrier 14 to the desired shape andpeeling it from the slide. The gage is then powdered, for example, withtalcum powder to keep it from sticking to itself.

The completed viscous low modulus strain gage 10 can be fastened to aworkpiece or specimen to be strain tested by any number of adhesives 22,for example, latex rubber, rubber cement, or Eastman 9-10 made byEASTMAN KODAK. Of course, the adhesive is selected dependent upon thematerial the gage 10 must adhere to. For the most uniform results, thegage should be firmly attached to the workpiece along the entire lengthof the gage-as shown in FIG. 3.

It is obvious that the strain gage 10 can be fabricated from differentmaterials than used in the preferred embodiment. It is preferred thatthe gage carrier 12 be any elastomeric non-conductive epoxy or polymercompound which has a modulus of elasticity of at least 60 psi and lessthan 1,000 psi. For example, a carboxy terminated polybutadiene (U.S.Pat. No. 3,305,523 Col.2, lines 3-9) with an amine curative and ahydrated silica filler (sold by Minnesota Mining and Manufacturing Co.under the designation EC-1949) is also suitable for the gage carrier 12and the elastomeric compound base 14 of the active gage element 16.EC-l949 comprises 100 parts per weight polybutadiene polymer, six partsby weight of nitrilotriethyl-betapropyleneaminobutryate and 60 parts byweight of hydrated silica.

The active gage compound 14 can be impregnated with any of the noblemetals 15 for example, silver, gold or platinum alloys or otherconductive metal particles. While the use of an ammonia water solutionis dis closed to aid in the mixing and application of the active straingage element other vehicles or solvents such as toluene, naphtha or acolloidal suspension of 1% lecithin in water may be employed.

The range of metal particle 15 size can be from about 5 microns to 75microns. The preferred range is from 5 to 45. Obviously, the smaller theparticle size, the more intimate the electrical contact is betweenparticles. Finally, the encapsulating material 21 can be anynonconductive epoxy or polymer compound having the desired elastomericproperties. The proportion of metal particles in the elastomericmaterial forming the active gage material should be from 50 to 90 weightpercent. A preferred range would be 60 to weight percent of metalparticles.

FIG. 3 is a section taken along lines 3-3 of FIG. 1 which clearlyillustrates the different layers making up the gage 10. Starting withthe test specimen 11, the gage carrier 12 is bonded along its entirelength by a suitable adhesive 22 previously described. The active gageelement 16 is made up of viscous carrier 14 which is impregnated withconductive particles preferably of a noble metal.

The wires 17 are coated and bonded to the element 16 by a conductiveadhesive composition 20 at ends 18 and 19 followed by an encapsulatingnon-conductive rubbery substance 21, thereby completing the assembly.The preferred range of thicknesses of layers l2, l6 and 21 are 0.001 to0.010 inch, 0.005 to 0.010 inch and 0.005 to 0.010 inch, respectively.

In operation, the low modulus gage is capable of at least 100 percentelongation with an electrical resistive type ofresponse proportionate tothe extent of elongation of the gage. Since the gage is extremely weakwhen compared to the relative rigidity of the test specimen, the gagecannot significantly contribute to or interfere with the true strainemanating from the test specimen therefore the readout instrumentationwill record accurate specimen responses.

Although particular embodiments have been chosen to best illustrate theadvantages of this invention, it is to be understood that the scope ofthe invention is not to be limited thereby:

We claim:

1. A viscous strain gage comprising;

an electrically insulative elastomeric base portion,

a bead of an electrically conductive elastomeric compound, wherein saidcompound comprises from 50 to 90 weight percent of conductive metalparticles, said compound being affixed to said base portion, andextending lengthwise thereof, said base portion and said elastomericcompound having a modulus of elasticity of from 60 psi to 1,000 psi, and

electrical connection means attached at two longitudinally displacedpoints on said conductive compound for conducting a signal indicative ofstrain on said gage to a strain measuring means.

2. The invention of claim 1 wherein the conductive elastomeric compoundcomprises noble metal particles embedded in a latex rubber material.

3. The method of fabricating a low modulus viscous strain gagecomprising the steps of;

dipping a mandrel into a liquid elastomeric solution for a length oftime to form a gage base portion, drying said base portion, mixing aquantity of conductive metal particles into liquid elastomer to form ahomogeneous conductive compound,

applying a bead of said conductive compound to said gage base portion,

attaching a pair of electrical leads to both ends of the bead of saidconductive compound with a metalparticle-containing adhesive therebyassuring electrical conductivity and compatability,

immersing said gage base, conductive compound,

conductive leads and reinforcing adhesive into a liquid elastomericsolution to provide an encapsulating layer, and,

heating said strain gage to cure the completed gage.

4. A viscous strain gage comprising;

an electrically insulative elastomeric base portion, a head of anelectrically conductive elastomerlc compound affixed to said baseportion, and extending lengthwise thereof, wherein said conductivecompound comprises 50 parts by weight of latex, parts by weight ofsilver powder, and 50 parts by weight of 10 percent ammonia solution,and

electrical connection means attached at two longitudinally displacedpoints on said conductive compound for conducting a signal indicative ofstrain on said gage to a strain measuring means.

1. A viscous strain gage comprising; an electrically insulativeelastomeric base portion, a bead of an electrically conductiveelastomeric compound, wherein said compound comprises from 50 to 90weight percent of conductive metal particles, said compound beingaffixed to said base portion, and extending lengthwise thereof, saidbase portion and said elastomeric compound having a modulus ofelasticity of from 60 psi to 1,000 psi, and electrical connection meansattached at two longitudinally displaced points on said conductivecompound for conducting a signal indicative of strain on said gage to astrain measuring means.
 2. The invention of claim 1 wherein theconductive elastomeric compound comprises noble metal particles embeddedin a latex rubber material.
 3. The method of fabricating a low modulusviscous strain gage comprising the steps of; dipping a mandrel into aliquid elastomeric solution for a length of time to form a gage baseportion, drying said base portion, mixing a quantity of conductive metalparticles into liquid elastomer to form a homogeneous conductivecompound, applying a bead of said conductive compound to said gage baseportion, attaching a pair of electrical leads to both ends of the beadof said conductive compound with a metal-particle-containing adhesivethereby assuring electrical conductivity and compatability, immersingsaid gage base, conductive compound, conductive leads and reinforcingadhesive into a liquid elastomeric solution to provide an encapsulatinglayer, and, heating said strain gage to cure the completed gage.